Antireflective polymer films (“AR films”) are becoming increasingly important in the display industry. New applications are being developed for low reflective films applied to substrates of articles used in the computer, television, appliance, mobile phone, aerospace and automotive industries.
The physical principles by which anti-reflection films and coatings function are well known. Several overviews can be found, for example, in Optical Engineering, S. Muskiant Ed, Vol. 6., Optical Materials, Chap. 7, p 161, 1985 and as shown in U.S. Pat. No. 3,833,368 to Land, et al. AR films are preferably constructed of alternating high and low refractive index (“RI”) polymer layers of the correct optical thickness. With regards to visible light, this thickness is on the order of one-quarter of the wavelength of the light to be reflected. The human eye is most sensitive to light around 550 nm. Therefore it is desirable to design the low and high index coating thicknesses in a manner which minimizes the amount of reflected light in this optical range. Desirable product features in AR films for use on optical goods are a low percentage of reflected light (e.g. 1.5% or lower) and durability to scratches and abrasions. These features are obtained in AR constructions by maximizing the delta RI between the polymer layers while maintaining or improving other critical material properties such as low coefficient of friction, high hardness and strong adhesion between the polymer layers. In addition to these types of performance features, it is necessary to process these materials by an economically favorable manufacturing process. Although inorganic materials, such as indium tin oxide (“ITO”), possess both high index and hardness, they are difficult and expensive to process into continuous films. Often times these materials require vacuum or chemical vapor deposition techniques. Moreover such metalized surfaces often reflect blue light and therefore optical substrates with such materials are slightly colored and therefore have compromised viewing cosmetics. In order to improve on these processing limitations of high index metal surfaces, new polymeric materials based on polycarbonate or polyesters can be used. However these materials do not have as high of refractive index as metalized surfaces and therefore there is a need for improved low refractive index materials with improved durability. Such materials can be used in conjunction with high index polymers to maximize the delta refractive index between the layers and minimize the amount of reflected light.
As described in Groh and Zimmerman, Macromolecules, Vol. 24 p. 6660 (1991), it is known that fluorine containing materials have an inherently low refractive index and are therefore useful in AR films. Fluoropolymers provide additional advantages over conventional hydrocarbon-based materials such as relatively high chemical inertness (in terms of acid and base resistance), dirt and stain resistance (due to low surface energy) low moisture absorption, and resistance to weather and solar conditions. However, fluoropolymers tend to have relatively low hardness and poor abrasion and wear resistance properties compared to hydrocarbon polymers such as polymethylmethacrylate (“PMMA”).
The refractive index of fluorinated polymer coatings is generally dependent upon the volume percentage of fluorine contained within the coating layer. Increased fluorine content in the layers typically decreases the refractive index of the coating. Several examples of AR coatings using fluoropolymers and fluorine containing materials can be found in the invention of Fung and Ko (U.S. Pat. No. 5,846,650), Savu (U.S. Pat. No. 5,148,511), Choi et al (U.S. Pat. No. 6,379,788), and Suzuki (U.S. Pat. No. 6,343,865), which are herein incorporated by reference. Although it is desirable to increase the fluorine content of the low refractive index coating in order to decrease the refractive index, an increase in fluorine content of the low index coating composition tends to decrease the surface energy of the polymer, which in turn can result in poor coating and optical cosmetic properties. Furthermore, low surface energy polymers can reduce the interfacial adhesion between the low refractive index layer and a high refractive index layer. A loss in interfacial adhesion between these layers will compromise the AR film durability.
The use of interpenetrating or semi-interpenetrating polymer networks between fluoropolymers and acrylate monomers have been previously described, for example, in EP 570254 (Kumar et al.) and WO9406837 (Bogaert et al.), which is herein incorporated by reference, for use in stain resistant, flexible, high gloss ultraviolet radiation curable floor coatings. The fluoropolymer component of the floor coating provides excellent weatherability, high temperature performance and stain resistant properties, while the introduction of the acrylate monomers aid in adhering the polymer material to the vinyl substrates, therein improving the durability of the coating. Further, the acrylate monomers improve the hardness of the resultant coatings. While these coatings are ideally suited for floor coatings, they have never been investigated for use in antireflection film layers.
Thus, it is highly desirable to form a low refractive index layer for an antireflection film having increased fluorine content, and hence lower refractive index, while improving interfacial adhesion to accompanying high index layers or substrates. The resultant AR film thereby has improved abrasion resistance as compared with low refractive index coatings formed in accordance with the prior art described above.